Re: [PATCH perf/core] perf intel-pt: Fix clang build failure in intel_pt_synth_pebs_sample

Gustavo A. R. Silva
2020-05-14T22:04:59Z

On Thu, May 14, 2020 at 12:06:48PM -0700, Ian Rogers wrote:
> On Thu, May 14, 2020 at 8:01 AM Gustavo A. R. Silva
> <gustavoars@kernel.org> wrote:
> >
> > On Thu, May 14, 2020 at 10:10:30AM -0300, Arnaldo Carvalho de Melo wrote:
> > > Em Wed, May 13, 2020 at 06:47:38PM -0500, Gustavo A. R. Silva escreveu:
> > > > Fix the following build failure generated with command
> > > > $ make CC=clang HOSTCC=clang -C tools/ perf:
> > > >
> > > > util/intel-pt.c:1802:24: error: field 'br_stack' with variable sized type 'struct branch_stack' not at the end of a struct or class is a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
> > > > struct branch_stack br_stack;
> > > > ^
> > > > 1 error generated.
> > > >
> > > > Fix this by reordering the members of struct br.
> > >
> > > Yeah, I noticed that as far back as with ubuntu 16.04's clang:
> > >
> > > clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/final)
> > >
> > > util/intel-pt.c:1802:24: error: field 'br_stack' with variable sized type 'struct branch_stack' not at the end of a struct or class is a GNU
> > > extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
> > > struct branch_stack br_stack;
> > > ^
> > > 1 error generated.
> > >
> > >
> > > Will fold this with the bug introducing the problem to avoid bisection
> > > problems.
> > >
> >
> > I agree. Also, the commit hash of the "Fixes" tag only applies to the
> > perf/core branch and, I guess that might create confusion.
>
>
> So while this fixes the warning I believe it breaks the intent of the code.
>
> tools/perf/util/branch.h:
> struct branch_stack {
> u64 nr;
> u64 hw_idx;
> struct branch_entry entries[];
> };
>
> tools/perf/util/intel-pt.c:
> struct {
> struct branch_stack br_stack;
> struct branch_entry entries[LBRS_MAX];
> } br;
>
> The array in br is trying to extend branch_stack's entries array. You
> might have to do something like:
>
> alignas(alignof(branch_stack)) char storage[sizeof(branch_stack) +
> sizeof(branch_entry) * LBRS_MAX];
> struct branch_stack *br = &storage;
>
> malloc/free may be nicer on the eyeballs.
>

Yep, I'd go for zalloc/free. There are a couple of places where dynamic
memory is being allocated for struct branch_stack:

tools/perf/util/cs-etm.c-256- if (etm->synth_opts.last_branch) {
tools/perf/util/cs-etm.c:257: size_t sz = sizeof(struct branch_stack);
tools/perf/util/cs-etm.c-258-
tools/perf/util/cs-etm.c-259- sz += etm->synth_opts.last_branch_sz *
tools/perf/util/cs-etm.c-260- sizeof(struct branch_entry);
tools/perf/util/cs-etm.c-261- tidq->last_branch = zalloc(sz);

tools/perf/util/thread-stack.c-148- if (br_stack_sz) {
tools/perf/util/thread-stack.c:149: size_t sz = sizeof(struct branch_stack);
tools/perf/util/thread-stack.c-150-
tools/perf/util/thread-stack.c-151- sz += br_stack_sz * sizeof(struct branch_entry);
tools/perf/util/thread-stack.c-152- ts->br_stack_rb = zalloc(sz);

there is even function intel_pt_alloc_br_stack().

Just out of curiosity, why the need of such a hack in this case (the
on-stack extension of branch_stack's entries array)?

Thanks
--
Gustavo